This invention relates to system for sand blasting using water as a propellant for sand. In one aspect, the invention relates to sand blast apparatus. In another aspect, the invention relates to a method for combining sand and water to form a surface abrading jet.
Pressurized water has been used previously as a propellant in said blasting. Water type sand blasting generally allows more rapid and efficient surface abrasion, or cleaning, than air type sand blasting.
Water type sand blasting systems generally include a pressurized water source, a sand supply container usually called a sand pot, a nozzle assembly for combining and spraying the sand and water, and supply conduits to convey sand and water from their sources to the nozzle assembly. Pressurized water sand blast systems have used a vacuum provided by venturi action in the nozzle assembly to draw air into the sand supply conduit adjacent the sand storage pot to fluidize and convey sand from the sand pot into the nozzle assembly for mixture with the water. These prior systems have several difficulties such as (1) plugging of the sand supply conduit adjacent the nozzle and (2) lack of flexibility in adjustment of the sand flow rate.
When sand in the supply conduit is wet the conduit may become plugged. This sometimes occurs when a sufficiently high vacuum is not produced becasue of insufficient venturi action in the nozle assembly. When this assembly lacks sufficient vacuum to draw in enough air to fluidize the sand, the sand simply remains static in the sand supply conduit. Any water present at the time in the nozzle assembly can backflow into said supply conduit, wetting the sand adjacent the nozzle and causing plugging.
Prior sand blast devices have generally introduced sand into the housing of the nozzle through a sand inlet which enters the housing at a position normally below the point where water is introduced into the housing. When such devices are shut off or water flow is very slow, water in the housing may flow into the sand conduit by the force of gravity, thereby wetting the sand and causing plugging.
Because of inefficiencies in the vacuum producing arrangements utilized, some previous sand blast systems have resorted to the use of extremely high pressure water, as high as 10,000 psi and above, to obtain sufficient vacuum to draw the desired amount of sand from the sand storage pot through the sand supply conduit into the nozzle assembly. Extremely high pressure water is costly to use, and such prior devices have often proved inefficient in their cleaning or abrading action. There is thus a need for a water type sand blast system which (a) provides the necessary operating vacuum in the venturi section of a sand blast nozzle, (b) does so using as low pressure water as possible, and (c) achieves a high surface abrading and cleaning rate compared to conventional sand blast systems.
In previous sand blast systems it has been difficult to provide a sand flow rate through the sand supply conduit accurately controllable at a level desired for a particular use. Previous systems have not consistently provided either a relatively uniform and constant sand flow rate or the desired absolute sand flow rate for a particular applicationn of a system.
The sand blast apparatus and method of the present invention overcome these and other problems found in previous water type sand blast systems.